This invention relates to flowmetering equipment and, more particularly, to improved bluff body flowmeters in which a sensor is located in an upstream surface of the body.
The natural phenomenon of vortex shedding has for a number of years been exploited in the flowmetering industry in order to measure flowrate. A simple count of vortices shed is all that is required to establish total flow. However, until recently attempts to design a vortex shedding flowmeter were unsuccessful because the shedding action was generally neither strong enough nor regular enough over flowrate ranges of industrial interest to permit reliable detection. Even in cases where the frequency was detectable, its relationship to flowrate was nonlinear and thus required complex and expensive electronic detection equipment.
This situation changed dramatically, however, with the advent of the Rodely bluff body flowmeter. As described in U.S. Pat. No. 3,572,117, assigned to the assignee hereof, there is a family of Rodely bluff body configurations which produce strong vortex shedding, free of intermittency, in fluid flowing through a conduit. In a preferred embodiment the Rodely bluff body has a planar base surface normal to the direction of fluid flow and a pair of converging downstream surfaces. To achieve nonintermittency the dimensions of the bluff body relative to the diameter of the conduit satisfy certain prescribed conditions set forth at column 3, lines 55-58.
Although the Rodely patent discloses a flowmeter in which the sensor is located external to and downstream of the bluff body, an important improvement incorporates an upstream sensor located in the base surface of the bluff body. One such configuration using a pair of such sensors in a differential detection scheme is taught by C. L. McMurtrie in U.S. Pat. No. 3,587,312 also assigned to the assignee hereof. Locating the sensor in the upstream base surface yields high signal-to-noise ratios because the sensor is not adversely affected by the turbulent wake of the bluff body.
One disadvantage, however, of sensing on the upstream base surface is the vulnerability of the sensor to damage caused by the impact of particles in the flowing fluid stream. Another disadvantage is the need to use an expensive matched pair of sensors and attendant electronics (filters, compensation, etc.) in order to obtain common mode rejection of in-phase fluid mechanical noise as in the McMurtrie flowmeter above.